All of the references cited herein are incorporated by references in their entirety.
MR angiography (MRA) that is based on the time-of-flight (TOF) contrast provides detailed anatomy of arterial vasculature and is routinely used in clinical brain imaging. As a complementary vascular imaging modality, MR venography (MRV) that is based on blood oxygenation level dependent (BOLD) contrast has been used clinically to delineate venous vascular anatomy in the brain.
Because MRA and MRV depict different neuronal and vascular abnormalities in brain diseases, it is desirable to acquire both MRA and MRV in clinical brain imaging studies. Nevertheless, both MRA (based on TOF contrast) and MRV (based on BOLD contrast) require relatively long scan duration, typically 5-15 minutes for each method. Accordingly, acquisition of both MRA and MRV in routine clinical brain imaging studies would prolong the total imaging time, reduce the MR examination throughput, and limit patient compliance. As a result, MRV is not routinely performed in clinical brain imaging examinations.
Recent studies have reported technical developments relating to simultaneous acquisition of both (TOF-based) MRA and (BOLD-based) MRV using the scan time required for the acquisition of only one, MRA or MRV. Du and Jin, Magnetic Resonance in Medicine, 59: 954 (2008); Barnes et al., Proc. Int'l. Soc. Mag. Reson. Med., 16: 2231 (2008). Despite this considerable advance, however, technical challenges remain in simultaneous acquisition of MRA and MRV due to conflicting scan conditions required for the optimization of MRA and MRV. On one hand, MRA necessitates the application of a ramped excitation pulse with higher flip angle, magnetization transfer contrast (MTC) pulse, spatial pre-saturation pulse, and shorter echo time (TE) for better inflow enhancement. On the other hand, MRV requires a flat excitation pulse with lower flip angle, no preparation pulse, longer TE, and low acquisition bandwidth for better T2* contrast.
These conflicting scan parameter requirements for the optimization of MRA and MRV image quality are not readily counterbalanced or reconciled in conventional methods for simultaneous acquisition of MRA and MRV. For example, in the dual-echo method proposed by Du and Jin (2008), supra, the radio frequency (RF) pulse conditions (i.e., excitation RF profile, flip angle, spatial presaturation pulse, MTC pulse) were not adjustable between the acquisitions of MRA and MRV. Consequently, the vascular contrast of the MRA and MRV could not be optimized.